1. Field of the Invention
The field of the invention is that of solid-state laser gyros used as inertial control unit. This type of equipment is used especially for aeronautical applications.
The laser gyro, developed some thirty years ago, is widely used on a commercial scale at the present time. Its principle of operation is based on the Sagnac effect, which induces a frequency difference Δν between the two optical transmission modes that propagate in opposite directions, called counterpropagating modes, of a bidirectional laser ring cavity undergoing a rotational motion. Conventionally, the frequency difference Δν is equal to:Δν=4AΩ/λLwhere: L and A are the length and the area of the cavity, respectively; λ is the laser emission wavelength excluding the Sagnac effect; and Ω is the rotation speed of the assembly. The value of Δν measured by spectral analysis of the beat of the two emitted beams is used to determine the value of Ω very accurately.
2. Description of the Prior Art
It may also be demonstrated that the laser gyro operates correctly only above a certain rotation speed needed to reduce the influence of intermodal coupling. The rotation speed range lying below this limit is conventionally called the blind zone.
The condition for observing the beat, and therefore for the operation of the laser gyro, is the stability of the intensities emitted in the two directions. This is not a priori an easy thing to achieve because of the intermodal competition phenomenon, which means that one of the two counterpropagating modes may have a tendency to monopolize the available gain, to the detriment of the other mode.
This problem is solved in standard laser gyros by the use of a gaseous amplifying medium, generally a helium/neon mixture operating at room temperature. The gain curve of the gas mixture exhibits Doppler broadening due to the thermal agitation of the atoms. The only atoms capable of delivering gain to a given frequency mode are thus those whose velocity induces a Doppler shift in the apparent frequency, which brings the atom to resonance with the mode in question. Forcing the laser emission to take place other than at the center of the gain curve by piezoelectric adjustment of the optical path length ensures that the atoms at resonance with the cavity have a non-zero velocity. Thus, the atoms that can contribute to the gain in one of the two directions have velocities opposite those of the atoms that can contribute to the gain in the opposite direction. The system therefore behaves just as if there were two independent amplifying media, one for each direction. Since intermodal competition has thus disappeared, stable and balanced bidirectional emission occurs. In practice, to alleviate other problems, a mixture consisting of two different neon isotopes is used.
However, the gaseous nature of the amplifying medium is a source of technical complications when producing the laser gyro especially because of the high gas purity required and of premature wear during its use, which wear is in particular due to gas leakage and to deterioration of the electrodes by the high voltages used to establish the population inversion.
At the present time, it is possible to produce a solid-state laser gyro operating in the visible or the near infrared using, for example, an amplifying medium based on neodymium-doped YAG (yttrium aluminum garnet) crystals instead of the helium/neon gas mixture, the optical pumping then being provided by diode lasers operating in the near infrared. It is also possible to use, as amplifying medium, a semiconductor material, a crystalline matrix or a glass doped with ions belonging to the class of rare earths (erbium, ytterbium, etc.). Thus, all the problems inherent with the gaseous state of the amplifying medium are de facto eliminated. However, such a construction is made very difficult to achieve due to the homogeneous character of the broadening of the gain curve of the solid-state media, which induces very strong intermodal competition and because of the existence of a large number of different operating regimes, among which the non-frequency-locked intensity-balanced bidirectional regime, called the “beat regime” is one very unstable particular case (N. Kravtsov and E. Lariotsev, Self-modulation oscillations and relaxations processes in solid-state ring lasers, Quantum Electronics 24 (10), 841-856 (1994)). This major physical obstacle has greatly limited hitherto the development of solid-state laser gyros.
To alleviate this drawback, one technical solution consists in attenuating the effects of the competition between counterpropagating modes in a solid-state ring laser by introducing optical losses into the cavity that depend on the direction of propagation of the optical mode and on its intensity. The principle is to modulate these losses by a feedback device according to the difference in intensity between the two transmitted modes in order to favor the weaker mode to the detriment of the other, so as constantly to slave the intensity of the two counterpropagating modes either to a common value or to a constant difference. Technically, the production of the feedback device may be based on the combination of three optical devices that act on the polarization state of the optical modes. These three devices are a linear polarizer, a reciprocal rotator or a waveplate, and a nonreciprocal rotator (French patent application 03/03645).